DC--DC converters have long been utilized in a variety of electronic devices. Such DC--DC converters often utilize isolation transformers coupled with a controlled converter primary switching circuit for supplying alternating pulses through the isolation transformer and a converter rectification and filtering secondary circuit.
A variety of transformer isolated D.C.--D.C. converters employ diodes to perform signal rectification. In lower voltage applications Schottky diodes are commonly utilized for signal rectification in the converter secondary circuitry. This is because Schottky diodes have a relatively low forward conduction voltage drop of about 0.3 volts. D.C.--D.C. converters employing diode rectification in the secondary circuit are well known and are well described in the literature. However, the forward voltage threshold of approximately 0.3 volts in Schottkey diodes still results in substantial losses in power conversion efficiency, particularly in power supplies having a desired output voltage of about 3.3 volts.
D.C.--D.C. converters are commonly utilized to power integrated circuit electronics. Such integrated circuit electronics typically require a drive voltage of either 3.3 or 5 volts. In order to enhance converter efficiency the voltage drop present in Schottky diodes is desirably avoided in such low voltage DC to DC converters. One proposal for avoidance of the use of Schottky diodes is presented in a publication entitled "The Performance Of The Current Doubler Rectifier With Synchronous Rectification" by Laszlo Balogh, HFPC, May 1995 Proceeding, pg.216. This publication proposes the use of a current doubler rectifier secondary in D.C.--D.C. converters in place of known push-pull, half bridge, and bridge topologies. The publication further proposes to use synchronous rectification to increase converter efficiency in low voltage, current doubler converters by replacing the Schottky diodes with control driven MOS-FETs'. These transistors, according to the publication should be switched on before the conduction of the MOS-FETs body diodes, while avoiding a short circuit across the secondary winding which may be caused by two simultaneously conducting synchronous switches. Thus, the above-mentioned publication proposes to utilize control-driven MOS-FETs in a D.C.--D.C. converter having a current doubler rectifier secondary.
It is clear that the current doubler rectifier employed in the Balogh publication is intended to employ a common input and output ground in order to avoid complex gate drives schemes. Thus, the converter proposed in the Balogh reference cannot provide complete transfer isolation without a complex gate drive scheme. This is primarily due to the reference's avoidance of a center tap in the transformer employed with the current doubler circuitry in order to avoid the complication of a split secondary transformer.
Because of the lack of such a center tap, the voltage output from the transformer secondary in the current doubler circuit proposed by the Balogh reference is too high to feed the gating circuitry used to gate the rectifying MOS FETs. While the secondary transformer voltage may be voltage divided to the desired voltage level, this results in power loss, deteriorating the efficiency of the current doubler rectifier of the Balogh publication. In the circuitry contemplated by the Balogh publication, transistor gating circuit power is likely obtained from the circuit primary since, in the Balogh circuit, a common input and output ground is utilized to avoid such complex gate drive schemes. Thus, the Balogh publication utilizes a driving technique having substantial disadvantages if full isolation between converter primary and secondary circuits is to be achieved.
Half bridge rectifiers such as illustrated in Prior Art FIG. 1 of the present application have also been known. FIGS. 1(a)-(c) of the present application illustrate a Prior Art isolated DC--DC converter which employs a half bridge or push pull primary circuit and a full wave secondary circuit employing Schottky diodes D.sub.1,D.sub.2. Converters of this type utilize the first and second rectifying diodes D.sub.1,D.sub.2 not only as rectification diodes, but as fly-back diodes as well. This is best understood by an examination of the operation of the FIG. 1 circuitry.
The circuit of FIG. 1 operates in three primary modes illustrated in FIGS. 1(a)-1(c), respectively. A first primary transistor Q.sub.1 which, in the preferred embodiment is an MOS-FET is turned on in a manner that is well known. When the first primary transistor Q.sub.1 conducts, current flows between the positive and negative (+,-) terminals of the input supply voltage V.sub.IN through the conductive first primary transistor Q.sub.1, a primary winding TR.sub.1 P of isolation transformer TR.sub.1, and second ripple filtering capacitor C.sub.2. This current is transferred across the core of the transformer TR.sub.1 to a first isolation transformer secondary winding TR.sub.1 S.sub.1 where it is supplied to a load R.sub.L through the first rectifying and fly-back diode D.sub.1 and a low-pass filter including filtering or smoothing inductor L.sub.1 and secondary filtering capacitor C.sub.3. Thus, power is supplied to the load.
When the first primary transistor Q.sub.1 is switched off, the first rectifying and fly-back diode D.sub.1 continues to conduct due to the free-wheeling action of the filtering or smoothing inductor L.sub.1. At this time, when both of the first and second primary transistors Q.sub.1,Q.sub.2 are turned off, the second rectifying and fly-back diode D.sub.2 also begins to conduct as illustrated in FIG. 1(b). At this time, both diodes D.sub.1 and D.sub.2 are operating as fly-back diodes, supplying the residual energy stored in the filtering or smoothing inductor L.sub.1 to the load. Thus, the diodes D.sub.1,D.sub.2 operate in conjunction with the filtering or smoothing inductor L.sub.1 to form a free-wheeling or fly-back path through which the current within the inductor L.sub.1 can "free-wheel".
Subsequently, the second primary transistor Q.sub.2 is switched on, conducting current from the input supply voltage V.sub.in through capacitor C.sub.1, the isolation transformer primary TR.sub.1 P, and the second primary transistor Q.sub.2. This induces current along a loop including the second isolation transformer secondary TR.sub.1 S.sub.2, through the center tap CT of the secondary, the load R.sub.L, the filtering or smoothing inductor L.sub.1, and the second rectifying and fly-back diode D.sub.2. Once again, the filtering or smoothing inductor L.sub.1 and secondary filtering capacitor C.sub.3 function to low-pass filter this output voltage, smoothing it into a more nearly constant voltage V.sub.0. When transistor Q.sub.2 again becomes non-conductive, diodes D.sub.1 and D.sub.2 operate as fly-back diodes transferring the current from the filtering or smoothing inductor L.sub.1 to the load R.sub.L.
The converter of FIG. 1 exhibits the known efficiency problems of converters utilizing Schottky diodes for signal rectification in the secondary side of the converter, as mentioned above.
Applicants of the present application have discovered that DC--DC converters employing full wave secondary rectifying circuits are substantially advantageous over current doubler secondary circuits of the type disclosed in the Balogh publication when both high efficiency and full isolation is desirable. This is because such full-wave rectification secondary circuits employing a split secondary winding transformer exhibit intermediate voltages at the transformer secondary both accessible and of a level desirable for gate circuit drive, which voltages are not present in the current doubler circuitry of the aforementioned Balogh publication.
While the use of a current doubler secondary circuit of the type proposed by the Balogh publication produces efficient D.C. to D.C. conversion, the Balogh secondary may not be easily and efficiently gated by circuitry powered by the converter secondary circuit which is fully isolated from the converter primary circuit. However, Balogh considers the use of a full wave secondary to be distinctly inferior to use of a current doubler rectifier.
In a DC to DC converter having a full wave secondary and split transformer secondary windings designed to drive the electronic circuitry at a normal drive voltage of, for example, 3.3 or 5 volts, the output of either secondary winding of the transformer is of the voltage level desirable for supplying power to electronic circuitry. However, at low voltages, the forward voltage drop of the rectification diodes is undesirable. It is therefore desirable to employ synchronous gating in a D.C.--D.C. converter having a full wave secondary, as such a D.C.--D.C. converter can more easily obtain the desired drive circuitry supply voltages from the converter secondary circuit. For this reason, such a converter is preferable, particularly in applications which require complete isolation between the converter primary circuit and the secondary circuit and load. Thus, although the isolated full wave DC to DC converter of the present invention requires a transformer with a split secondary, a gating circuitry drive voltage of a desired level may be readily obtained across either of the secondary coils without substantial efficiency loss, while maintaining full primary/secondary isolation.